1. Field of the Invention
The present invention relates to the design of semiconductor light-emitting devices. More specifically, the present invention relates to a method for fabricating InGaAlN-based semiconductor light-emitting devices on carbon-based substrates.
2. Related Art
Solid-state light-emitting devices are expected to lead the next wave of illumination technologies. High-brightness light-emitting diodes (HB-LEDs) are emerging in an increasing number of applications, from light source for display devices to light-bulb replacement for conventional lighting. Meanwhile, solid-state lasers continue to beam as the driving force in many critical technological fields, from optical data storage, to optical communication networks, and to medical applications.
In particular, recent success in the development of nitride-based InxGayAl1-x-yN (0<=x<=1, 0<=y<=1) (InGaAlN-based hereafter) LEDs and lasers (e.g., GaN-based LEDs and lasers) not only extends the light-emission spectrum to the green, blue, and ultraviolet region, but also can achieve high light emission efficiency. InGaAlN-based semiconductor light-emitting devices have been widely used in applications including full-color large-screen displays, traffic lights, backlight source, and solid-state lighting.
Successful epitaxial growth of InGaAlN-based materials typically requires matching of the lattice constant and thermal-expansion coefficients of the substrate and epitaxial layers. Consequently, unconventional substrate materials, such as sapphire (Al2O3), are often used to grow InGaAlN materials in order to achieve such matching. Furthermore, these light-emitting devices are typically configured to have both positive and negative electrodes fabricated on the same side of the device.
Unfortunately, the above-described light-emitting devices often suffer from low utilization of the light-emitting material, low light-emission efficiency, and low thermal conductivity through the sapphire substrate. A further problem is associated with the fact that a typical Ohmic-contact layer can absorb some light during the emission process, and hence causes negative impact on the electro-optical property of the device. Moreover, sapphire substrates are typically expensive and are complex to fabricate, and can therefore lead to high manufacture costs.
Silicon (Si) substrates have low cost and can facilitate easy fabrication. Recent successes in research efforts have allowed semiconductor light-emitting structures to be fabricated on conventional Si substrates. However, if InGaAlN-based materials formed on Si substrates are used to fabricate light-emitting devices with the same-side electrode configuration, the following problems can arise: (1) Si substrate can absorb light emitted from active region of the device; (2) the same-side electrode configuration can reduce wafer utilization; and (3) P-type Ohmic-contact layer can also absorb some emitted light. On the other hand, if the device is fabricated with a vertical electrode configuration such that one of the two electrodes is formed directly on the Si substrate surface and below the light-emitting structure, the wafer utilization can be significantly improved. Unfortunately, the presence of an Aluminum Nitride (AlN) buffer layer above this bottom electrode can increase the operation bias voltage of the device, while the light absorption issues remain unsolved.
In recent years, researchers have been experimenting with wafer-bonding techniques to construct LEDs with vertical electrode configurations. During wafer bonding, a second support substrate with low resistance is bonded to the top of the InGaAlN-based LED multilayer structure fabricated on a Si growth substrate. The Si growth substrate is subsequently removed through wet etching, which effectively transfers the InGaAlN-based LED multilayer from the initial growth substrate to the new substrate. This process allows fabricating the InGaAlN-based LEDs which emit light through the N-type layer while having a vertical electrode configuration. Such vertical-configuration LEDs can improve light-emission efficiency, increase wafer utilization, and reduce serial resistance of the LEDs.
Note that after transferring the InGaAlN-based multilayer structure to the new substrate, the heat dissipation property of the new substrate can have significant impact on the quality of the final device. These new substrates are typically either Si substrates or metal substrates. Although Si substrates are easy to process, such as dicing and cutting, they have inferior thermal conductivity in comparison with single-element metals such as copper and silver, and some high thermal-conductivity alloys. On the other hand, even though metal substrates, such as copper and silver have high thermal conductivities, they suffer from other problems, such as being difficult to cut and having a mismatching thermal expansion coefficient with respect to the transferred InGaAlN-based layers.
Hence, what is needed is a new substrate which has high thermal conductivity and is easy to process for supporting the InGaAlN-based multilayer structure transferred from the initial growth substrate.